IP

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IP
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Classless Inter-Domain Routing

• Classful addressing scheme wasteful
• IP address space exhaustion
• A class B net allocated enough for 65K hosts
• Even if only 2K hosts in that network
• Solution CIDR
• Eliminate class distinction
• No A,B,C
• Keep multicast class D

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Classless Addressing

• Addresses allocated in blocks
• Number of addresses assigned always power of 2,
  and always on the boundary. That is, if 2048
  addresses, it will start with some address with
  all lower 11 bits being 0.
• Network portion of address is of arbitrary length
• Address format a.b.c.d/x
• x is number of bits in network portion of address

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Allocating Addresses

• Assume abundant addresses are available starting
  at 194.24.0.0.
• Cambridge university needs 2038 addresses, it is
  given 194.24.0.0 to 194.24.7.255. Mask
  255.255.248.0.
• Oxford need 4096 addresses. Because the
  requirement is that must be on the boundary, it
  is given 194.24.16.0 to 194.24.31.255. Mask
  255.255.240.0.
• Edinburg needs 1024 addresses, is given
  194.24.8.0 to 194.24.11.255. Mask 255.255.252.0.

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CIDR

• A router keeps routing table with entries
• IP address, 32-bit mask, outgoing line
• When an IP packet arrives, the router checks its
  routing table to find the longest match.

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CIDR

• Example.
• Cambridge 194.24.0.0/21 194.24.0.0 --
  194.24.7.255
• Edinburgh 194.24.8.0/22 194.24.8.0 --
  194.24.11.255
• (Available) 194.24.12.0/22 194.24.12.0 --
  194.24.15.255
• Oxford 194.24.16.0/20 194.24.16.0
  -- 194.24.31.255
• When a packet addressing to 194.24.17.4 arrives,
  where should it be sent to?
• And with all masks, find one that matches the
  longest.

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CIDR Entry aggregation

• How does a router in Tallahassee route packet to
  C,E and O, assuming that he has only two outgoing
  links?
• All to New York.
• Can he reduce the size of his routing table?

C
E
N
O
H
T
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CIDR Entry Aggregation

• From 194.24.0.0 to 194.24.31.255, all to N.
• So aggregate the three entries into one
  194.24.0.0/19.
• The N router can do the same thing.

C
E
N
O
H
T
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CIDR

• If later the free address space 194.24.12.0/22
  194.24.12.0 -- 194.24.15.255 is assigned to
  Pittsburgh and has to go through Houston, what
  should the router at Tallahassee do?

C
E
N
P
O
H
T
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CIDR

• When a packet arrives addressing 194.24.15.8, the
  router checks the routing table and there will be
  two matches 194.24.12.0/22 and 194.24.0.0/19.
  Pick the longest match.

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NAT Network Address Translation

• IP address is a scarce resource.
• So, give a company only one or a few IP addresses
  used by the gateway router.
• Within the company, each machine has an unique IP
  address, chosen from
• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16
• These addresses can only appear within a company
  but never on the outside Internet

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NAT

• Whenever a machine wants to send a packet to the
  outside, the packet will be sent to the NAT box.
• The NAT box will convert the internal IP address
  to the real IP address of the company, and pass
  the packet to the gateway router.
• When there is a packet destined for an internal
  machine arrived at the router, what should the
  router and NAT box do?
• For IP packets carrying TCP or UDP, use port
  number. Other protocols are much more
  complicated.

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NAT

• For IP packets carrying TCP or UDP, use port
  number.
• When an outgoing packet arrives at the NAT box,
• The IP address is replaced
• The source port number is replaced
• Header checksum is recomputed
• When a reply came for this process, use the
  replaced source port number as index to find the
  correct IP address and original port number.

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ICMP

• ICMP Internet Control Message Protocol
• Each ICMP message is encapsulated in an IP packet
• Treated like any other datagram, but no error
  message sent if ICMP message causes error
• Some interesting messages
• Time exceeded When an IP packet arrived at a
  router is dropped because the TTL field becomes
  0, the router will send an ICMP TIME EXCEEDED
  message back to the source. Used in traceroute.
• Echo and Echo reply ping.

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Address Resolution

• IP address is virtual
• Not understood by underlying the hardware of
  physical networks
• IP packets need to be transmitted by the
  underlying physical network
• Address resolution
• Translating IP address to physical address
• Address Resolution Protocol (ARP)

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ARP Example
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ARP Cache

• Each computer maintains a cache table
• IP address ? hardware address mapping
• Only about computers on the same network
• Exchanges ARP messages
• To resolve IP addresses with unknown hardware
  addresses

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ARP Protocol

• When a node sends an IP packet
• To another node on the same physical network
• Look up destination address in the ARP table
• If not found
• Broadcast a request to the local network
• Whose IP address is this?

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ARP Response

• The target node responds to sender (unicast?)
• With its physical address
• Adds the requester into its ARP table (why?)
• On receiving the response
• Requester updates its table
• Other nodes upon receiving the request
• Refresh the requester entry if already there
• No action otherwise (why?)
• Table entries deleted if not refreshed for a while

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DHCP

• DHCP Dynamic Host Configuration Protocol
• A new machine asks for an IP address
• Broadcast DHCP DISCOVER packet
• A DHCP relay agent got this packet and relay it
  to the DHCP server
• The DHCP server assigns an IP address
• Periodically renew

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Hierarchical Routing

• aggregate routers into regions, autonomous
  systems (AS)
• routers in same AS run same routing protocol
• intra-AS routing protocol
• routers in different AS can run different
  intra-AS routing protocol

• special routers in AS
• run intra-AS routing protocol with all other
  routers in AS
• also responsible for routing to destinations
  outside AS
• run inter-AS routing protocol with other gateway
  routers

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Intra-AS and Inter-AS routing

• Gateways
• perform inter-AS routing amongst themselves
• perform intra-AS routing with other routers in
  their AS

b
a
a
C
B
d
A
network layer
inter-AS, intra-AS routing in gateway A.c
link layer
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Intra-AS and Inter-AS routing
Host h2
Intra-AS routing within AS B
Intra-AS routing within AS A
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Why different Intra- and Inter-AS routing ?

• Policy
• Inter-AS admin wants control over how its
  traffic routed, who routes through its net.
• Intra-AS single admin, so no policy decisions
  needed
• Scale
• hierarchical routing saves table size, reduced
  update traffic
• Performance
• Intra-AS can focus on performance
• Inter-AS policy may dominate over performance

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Intra-AS Routing

• Also known as Interior Gateway Protocols (IGP)
• Most common IGPs
• RIP Routing Information Protocol
• OSPF Open Shortest Path First
• IGRP Interior Gateway Routing Protocol (Cisco
  proprietary)

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OSPF

• Represents the network as a graph, and runs the
  shortest path algorithm to find the path to any
  router.
• Divide the network into areas for scalability.
• The backbone area is called area 0
• Within one area, a router has the same link state
  database as all other routers.
• Route local area ? backbone ? local area

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OSPF

• Each router knows the shortest path to reach
  routers within his area.
• Backbone routers also accept information from
  area border routers to compute the shortest path
  to reach other routers. Then advertise this
  information to the border routers, who tells
  routers inside the area. To be able to select
  the best exit router in an area

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OSPF

• To learn the link state, use flooding
• select a designated router and let it to be
  adjacent to all other routers in the same area.
  Only exchange link state between the adjacent
  routers
• Messages include
• HELLO, LINK STATE UPDATE, LINK STATE ACK,
  DATABASE DESCRIPTION, LINK STATE REQUEST

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Inter-AS routing
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Internet Inter-AS routing BGP

• BGP (Border Gateway Protocol) the de facto
  standard
• Path Vector protocol
• similar to Distance Vector protocol
• each Border Gateway broadcast to neighbors
  (peers) entire path (I.e, sequence of ASs) to
  destination
• E.g., Gateway X may send its path to dest. Z
• Path (X,Z) X,Y1,Y2,Y3,,Z

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Internet Inter-AS routing BGP

• BGP messages exchanged using TCP.
• BGP messages
• OPEN opens TCP connection to peer and
  authenticates sender
• UPDATE advertises new path (or withdraws old)
• KEEPALIVE keeps connection alive in absence of
  UPDATES also ACKs OPEN request
• NOTIFICATION reports errors in previous msg
  also used to close connection

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Internet Inter-AS routing BGP

• Suppose gateway X send its path to peer gateway
  W
• W may or may not select path offered by X
• cost, policy (dont route via competitors AS),
  loop prevention reasons.
• If W selects path advertised by X, then
• Path (W,Z) W, Path (X,Z)
• Note X can control incoming traffic by
  controlling its route advertisements to peers
• e.g., dont want to route traffic to Z ? dont
  advertise any routes to Z

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BGP an example
3210 4210 7610
128.186.0.0/16